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So

Open-hearth Process,



separately removed The temperature must be exceedingly high
in order to preserve the iron in its fluid state after the expulsion
of the carbon; the entire absence of the latter is discovered by
the application of the spectroscope, this being the most practical use
of that most wonderful Instrument. The next operation is the
adding of so much carbon as is needed to produce the steel re-
quired, and this is done by putting into the converter a measured
amount of very pure cast iron called Spiegtleism* and mixing it
well with the metal by re-applying the blast for a short time. The
now converted steel is transferred to the ladle B, which is swung
round by the crane c, and the metal poured into the ingot through
the hole D on releasing the plug at the bottom of the ladle.

The ingots may be afterwards piled and rolled as previously
described, to produce a fibrous steel, and if used for forging and
welding purposes should not have too much carbon in their com-
position; or, if required for steel castings, may be re-melted in
suitable quantities, much as in the way already mentioned for
cast iron.

The Siemens-Martin, or open-hearth process, is carried on
in a special kind of furnace, called a regenerative furnace, invented
by Sir W. Siemens. Fig. 91 is a drawing which will shew all the
necessary parts. A is the hearth, sloped in the side elevation, so
that the metal may run out when tapped at T. A current of air is
allowed to pass under the hearth at c, to prevent the melting of
the iire-clay. The combustion of a mixture of common coal gas
and air is the source of heat, the arrows showing the passage to
the interior of the gas through the valve o, while the air enters
through the valve A. In the figure the mixture is seen entering
the right side of the furnace. Being ignited at j by means of a
red-hot bar, gradually and carefully at first, the flame is directed
by the roof on to the metal, and the heat passes away by the left
side of the furnace, returning through the valves and past the
damper D to the chimney. Were it not, however, for Siemens'
beautiful regenerative principle a great deal of heat would be
wasted. The regenerators are shewn at nurr; they are hajrd
fire-clay or silica bricks piled as a grating. The rejected heat
from the hearth is intercepted by those marked n R, so that
* See Appendix II, j>. 790.